Multi-layer laminates such as carbon fiber-reinforced composites (CFRP) and fiberglass composites (FRP) have become widely used in recent years in a large number of applications. These laminate materials most often consist of woven layers of strong fibers that are coated with resins and processed or cured to form a solid structure. Depending on the choice of the fibers and the resin systems used, these materials can be formulated and molded to produce components with excellent mechanical properties and unique geometries that would be difficult or impossible to obtain using other materials.
The properties of high strength CFRP materials may be widely varied by manipulating the characteristics of the matrix formulation, as well as the fiber type, content, orientation, build-up, and the methods used to shape these materials into a finished structure. This variability and the general strength of CFRP materials make them useful in a wide variety of applications, ranging from bicycle frames to aircraft components.
The reinforcing fiber most widely used in aircraft structures is a carbon fiber produced by the thermal decomposition of polyacrylonitrile (PAN). Such thermal decomposition converts the PAN fiber to a pure carbon fiber that is highly abrasive and very strong. In some specific examples, such carbon fibers are reported to have tensile strengths of about 800,000 psi and a modulus of about 40 million psi. In producing structures such as aircraft components, these high-strength fibers are first woven into thin sheets and combined with resins to form flat sheets of composite referred to as “prepregs.” Components such as composite skin sections of aircraft may be produced by placing multiple layers of such prepregs in molds and then using pressure and heat to shape and cure them into a complex wing surface. Alternatively, components may be constructed by chopping carbon fibers into shorter lengths and blending them with resins to produce a compound suitable for use in compression molding or resin-transfer molding.
CFRP laminate parts have been used in the manufacture of aircraft for several years. In one example, the 777 aircraft manufactured by Boeing uses CFRP for the passenger cabin floor beams, for the vertical and horizontal tails, and for aerodynamic fairings. Overall, CFRP-based components make up about 9% of the structural weight of this aircraft.
Composite components such as aircraft parts are joined together or to other materials by fasteners. Processes used to join such components generally include the steps of drilling and countersinking a precision hole in the structures to be joined and then inserting a close-fitting fastener in a secure manner. Drilling of CFRP components is difficult, however, as a result of the highly abrasive nature of the material and its tendency to delaminate and fray when processed by conventional drills. One of the more serious problems experienced in drilling CFRP occurs when the exit of the drill from the hole produced leaves uncut fibers exposed in the hole. Such fibers can then interfere with the proper fit of the fastener used to join the materials.
Currently, the problem of holes produced in CFRP having frayed and uncut fibers is resolved by subjecting the affected CFRP component to a manual fiber removal/hole clearing process prior to component assembly. Forced inclusion of such a manual step prevents the use of automated systems capable of drilling and fastening components in a single operation. This may greatly increase the manufacturing time and costs of items constructed with CFRP components.
In addition to the above, it is understood in the art that the drilling speeds and feed rates used with conventional drills in conventional materials are unsuitable in CFRP-based components. Unlike aluminum materials used in the construction of aircraft, CFRP resin materials generally have a low melting point. As a result, machining operations used with CFRP products must not exceed an operating temperature of more than a few hundred degrees Farenheit. In such systems, keeping the cutting edge cool is made difficult by the low thermal conductivity of the resin matrix of the CFRP. While chips produced in the machining of aluminum carry away heat, cuttings produced by machining advanced composite materials such as CFRPs carry away very little of the heat generated by the machining process. As a result, heat buildup in the cutting zone is common. Such an accumulation of heat may cause the resin to oxidize and/or degrade, thus increasing risk of delamination and decreasing the quality and shape of the hole produced in the component and increasing the likelihood of uncut fibers.
In addition to the above, the widely different material properties of the fiber and resin matrix encapsulating the fibers in CFRP components also render machining of the product difficult. Specifically, even slight dulling of currently-used tool cutting edges can cause delamination of CFRPs and displacement of fibers from their positions in the resin matrix of the CFRP. Even modest cutting forces can cause delamination or leave residual stresses. Reduced drilling speeds and feed rates are thus currently used in CFRP product processing to reduce the occurrence of uncut fibers and delamination. This reduction in drilling speed further adds to the costs of machining composite laminate components.
Alternatives to drilling advanced composites are known in the art. Such alternatives generally include orbital milling processes that are capable of producing clean exit holes in components made of advanced composites such as CFRP. Such an approach requires highly specialized machinery, however. This machinery cannot be used in many of areas of aircraft assembly and takes considerably greater time to form each hole. As with the prior art drilling and post-processing methods discussed above, such orbital milling approaches may greatly inflate the cost and time of machining.
As a result, it would be an improvement in the art to provide drills and methods of their use to produce holes in advanced composite materials such as CFRP that require no additional processing, and thus, which may be used with combination drilling-and-fastening operations to increase the speed and efficiency of component assembly. It would be a further improvement in the art to provide such drills with polycrystalline diamond cutting surfaces to provide increased tool life and use with a broader range of materials. Similarly, it would be beneficial to provide such a drill capable of being refurbished to further extend tool life and reduce manufacturing costs. It would be further beneficial to provide combination drills with countersinks for use with such methods and materials. Such drills and methods of their use are provided herein.